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Western U.S. withers In heat wave

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Extreme heat will continue in Southwest, but rain will drench Southeast

The Pacific Northwest will get a break from blazing temperatures

Arizona's massive wildfire could get rain, but also wind gusts and lightning

It'll be firecracker hot in the Southwest on the Fourth of July, but the Southeast and Ohio River Valley will be so drenched that floods are possible.

Meanwhile, the Northeast will get a reprieve from storms that left knee-deep flooding in some areas.

"The most widespread showers and thunderstorms on Thursday will be from the central Gulf Coast northward into the Ohio River Valley," the National Weather Service said. "There is a slight risk of excessive rainfall/flash flooding across much of the Southeast through Wednesday."

As the wet weather system moves up toward the Midwest, the relatively cool temperatures along the East Coast will give way to more summer-like conditions in the Mid-Atlantic and Northeast.

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East Coast faces wet and wild Fourth

Deadly wildfire burns in Arizona – A deadly wildfire leaves behind little but a burned-out car and the remains of a house in a Yarnell, Arizona, neighborhood on Wednesday, July 3. The fire started a week ago near Yarnell, apparently because of lightning strikes. Nineteen firefighters were killed Sunday, June 30, battling the blaze northwest of Phoenix.

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Deadly wildfire burns in Arizona – An aerial overview shows the devastation in Yarnell on July 3. Crews have begun making progress on the wildfire, which reportedly was 80% contained by Thursday night, July 4.

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Deadly wildfire burns in Arizona – Smoke from the Yarnell Hill Fire is visible from a distance on a road to Yarnell on Monday, July 1.

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Deadly wildfire burns in Arizona – Local firefighters embrace July 1 at a Prescott, Arizona, memorial service for the 19 firefighters killed in the Yarnell Hill Fire, northwest of Phoenix.

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Deadly wildfire burns in Arizona – A couple embraces during the Prescott memorial service for the fallen fighters on July 1. The elite team members' deaths on Sunday, June 30, marked the deadliest day for firefighters since the 9/11 attacks.

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Deadly wildfire burns in Arizona – Members of the community give a standing ovation as local firefighters arrive at the July 1 memorial service at Embry-Riddle Aeronautical University in Prescott.

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Deadly wildfire burns in Arizona – Toby Schultz lays flowers on July 1 at the fence of the fire station in Prescott, Arizona, the home base of 19 firefighters who died in the Yarnell Hill Fire.

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Deadly wildfire burns in Arizona – Flowers hang on the fence outside the Granite Mountain Hot Shot Crew fire station on July 1.

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When will record-breaking heat wave end?

But the East Coast warm-up will have nothing on the oppressive heat wave scorching the West Coast.

While the Pacific Northwest will get some relief, "extreme heat will continue across the Western states, especially the Southwest," the weather service said.

Cities in California, Nevada and Arizona have already suffered through heat topping 120 degrees in the past few days.

The heat was so torrid in Idaho that Boise residents could bake cookies without an oven.

CNN affiliate KTVB placed a pan of chocolate chip cookie dough on the dashboard of a car. Within a few hours, the cookies were fully baked -- and even overcooked.

Forecasters in Las Vegas noted that a similar heat wave in 2005 killed 17 people. They urged neighbors and relatives to check on those most susceptible to heat-related illness -- children, the elderly and the chronically ill.

Relief and trouble in Arizona

The extreme heat continues to wreak havoc for firefighters trying to stop Arizona's Yarnell Hill wildfire, which has scorched more than 8,400 acres, about 13 square miles of land.

The possibility of thunderstorms this week could drizzle much needed rain over the fire. But storms could also bring two serious dangers: wind gusts of up to 20 mph and lightning, the suspected cause of the inferno.